The length of the “fallen insect” season affects river ecosystems

Insects that fall from the surrounding forest provide seasonal food to the fish in the streams. Researchers at Kobe University and the University of Tokyo have shown that prolonging this period has a profound effect on food webs and ecosystem functions found in the streams.

These research results provide evidence that changes in forest seasonality also affect nearby river ecosystems. This finding highlights the importance of predicting the impacts of climate change on ecosystems.

The research group consisted of Associate Professor Sato Takuya and UEDA graduate student Roy from the University of Kobe Graduate School, and Associate Professor Takimoto Gaku from the Graduate School of Agricultural and Life Sciences at the University of Tokyo.

The results have been published in The Journal of Animal Ecology On March 4, 2021.

the main points

  • If terrestrial insects are only present in rivers for short and intense periods, this reduces competition for food among salmon. As a result, all types of salmon experience equal growth and there is little difference in size between individuals. However, if the same amount of terrestrial insects is available over a long period, this leads to a hierarchy in which the great salmon monopolizes the food supply of terrestrial insects. As a result, only larger fish grow larger and there is greater variation in the size of individual fish.
  • Whether terrestrial insects are available for an intense or prolonged period of time has a different effect on the numbers of benthic invertebrates (* 1) and the rate of leaf collapse (* 2). This result was only observed in experimental groups as it was easy for fish to consume the majority of benthic invertebrates.
  • This study demonstrated that the length of seasonal cycles has a domino effect on ecosystems. Climate change is causing major changes in the seasonal cycles of organisms. The results of this research indicate the importance of understanding and predicting the response of ecosystems to climate change.

Research background

The cool, clear flowing streams are home to many types of salmon including red trout, cherry salmon and Japanese char (hereafter referred to as “stream fish”). These stream fish prefer to eat terrestrial invertebrates that fall into the river from surrounding forests. When there are many of these land-dwelling insects in the water, stream fish tend not to eat the benthic invertebrates that inhabit the river, such as amphibians and young aquatic insects. This results in a sustainable large population of benthic invertebrates, which eat leaves that have fallen into the water. Thus, this large population in turn speeds up the rate of leaf decomposition in the river (leaf breakdown rate = ecosystem functions of the stream). Thus, the presence of terrestrial invertebrates alters the diet of fish, which has a significant effect (positive indirect effect (* 3)) on riverine food webs and ecosystem functions (Fig.1).

The amount of terrestrial invertebrates that end up in rivers increases as the trees grow new leaves in the spring, peaks in early summer, and then decreases when the trees lose their leaves in the fall. This seasonal pattern is common in streams located in temperate to temperate regions. However, the period between new leaf growth and defoliation is short in forests at high latitudes and elevations, but long in forests at lower latitudes and altitudes. Therefore, although there may be a similar total number of terrestrial invertebrates in rivers over the course of a year, they are likely to be present in water for periods of intense at high latitudes / altitudes and long periods at low latitudes / altitudes.

research aims

What kind of impact does the length of the supply of terrestrial invertebrates have on the food webs of streams and stream fish?

This research study investigated the effects of different supply periods on red trout (Oncorhynchus masou ishikawae), As well as the impact on food webs and ecosystems in streams.

Research methodology and results

Outdoor experiments were conducted in large ponds that simulate river ecosystems at Kyoto University’s Wakayama Forest Research Station. The experiments were conducted from August through November 2016, and the same total amount and type of terrestrial invertebrates (mealworms) were provided in each experiment over a period of 90 days. In the pulsed trial groups, concentrated quantities of mealworms were provided daily over a 30-day period in the middle of the 90-day experiment (i.e., 30 to 60 days), while the extended-trial groups were given a steady supply of mealworms for a third of the 90-day period (Pictures B and C). Control groups were also established that were not administered to terrestrial invertebrates. The following aspects were examined: salmonid stomach contents, body size, number of benthic invertebrates, and leaf crushing rate.

In the pulse groups, it was difficult for the larger fish to monopolize the mealworm supply because a large amount was given each time, so the smaller fish were also able to eat the mealworms (Fig.2a). After the experiment, it was found that there was little difference in size between fish in the legume groups (Fig.3), indicating that these conditions led to a community in which individual fishes were difficult to control the food supply. Conversely, in elongated groups, it was easier for the larger fish to monopolize the food flowing downstream, meaning that the smaller, outboard competing fishes would hardly eat any mealworms (Fig.2a). After the experiment, a large variation was found in the size of the fish in the elongated group (Fig. 3), and it was revealed that these conditions led to a community where large fish could easily monopolize the food supply. Moreover, individuals who attained mature size were found among the predominant fishes of the elongated group, which also indicates the effect on population growth.

In the pulse group where large and small fishes could eat mealworms, there was a significant decrease in the amount of benthic invertebrates that all fish eat compared to the control group (Fig.2b). On the other hand, young fishes tend to consume benthic invertebrates more frequently in elongated groups where larger fish have a monopoly on the flour worm supply (Fig.2b). Thus, there was no significant decrease in the amount of benthic invertebrates consumed in the elongated groups compared to the control group.

The numbers of benthic invertebrates were at their highest in the pulse groups as all salmon consumed fewer benthic invertebrates, resulting in the fastest collapse of fallen leaves. On the other hand, in elongated groups where young fish ate many benthic invertebrates, the number of these insects and the rate of leaf-breaking did not reach the levels seen in the pulse groups. In other words, the presence of terrestrial invertebrates changed the feeding habits of the fish, which had an indirect positive effect on benthic invertebrates and the rate of leaf breakage, and this effect was greater in the pulse groups than in the elongated groups. A large variation in the strength of this indirect effect is observed between the impulsive and extended groups when a large proportion of benthic invertebrates consist of midges, which are readily consumed by salmon. However, this effect was not observed when isopods, which are rarely found in salmon stomach contents, constitute a large proportion of benthic invertebrates.

The main reason behind this recent discovery is thought to be that it is difficult for the dietary habits of fish to influence the numbers of benthic invertebrates and the rate of leaf breakdown in such conditions. If it is difficult for fish to eat the majority of benthic invertebrates, their diet is unlikely to change from terrestrial invertebrates to benthic invertebrates.

More research

This research provides preliminary evidence that the length of time that forest-dwelling insects are found in rivers has a significant effect on salmon growth rate, size distribution, flowing food webs and ecosystem functions. In addition, the impact on the stream’s ecosystems is more pronounced when there are a large number of benthic invertebrate species that are easy for salmon to consume. These results demonstrate the vital importance of studying the seasonality of organisms, which link ecosystems such as those in forests and rivers, in order to understand food web structures and ecosystem functions.

Based on these research findings, we can see how climate change worldwide affects the seasonality of organisms living in certain ecosystems and that these changes, in turn, are likely to have a significant ripple effect on surrounding ecosystems. Investigating these aspects and being able to understand and predict the domino effect that climate change has on ecosystem behavior is an important issue in the study of microbiology.

Nowadays, researchers have set up observation sites all over Japan, from Hokkaido in the north to Kyushu in the south. They are conducting longitudinal observations of seasonal elevation and degradation of forest and river insects in cooperation with local researchers. Through a set of wide-ranging longitudinal observations of external species and experiences such as those in this study, they hope to deepen our understanding of how climate change affects seasonal aspects of ecosystems, with the goal of being able to predict these impacts.


Glossary of terms

1. Benthic invertebrates: the collective name for the thornless aquatic organisms that live in the benthic zone, the ecological zone at the lowest level of the water body. In the streams, this category includes young aquatic insects such as dragonflies and stoneflies, as well as amphibians and isopods.

2. The rate of leaf fragmentation: The speed at which leaves fall into streams is divided by their consumption by benthic invertebrates. This is an important function of an ecosystem to maintain the cycle of matter in flows.

3. Indirect positive effect: When terrestrial invertebrates are introduced into a watercourse, the main diet of fish changes from benthic invertebrates to benthic invertebrates, resulting in an increase in the numbers of benthic invertebrates and the rate of leaf litter breakage. The introduction of other organisms (terrestrial invertebrates) alters the interaction between the two species (the predator-prey relationship between fish and benthic invertebrates).


This research was supported by a KAKENHI grant from the Japanese Society for the Promotion of Science (JSPS, Grant No. JP 15H04422).

Journal information


“Effects of Duration of Resource Subsidy in a Waste Based Ecosystem: A Median World Experience”?

DOI: 10.1111 / 1365-2656.13440

The Authors:

Takuya Sato, Rui Ueda, and Jaco Takemoto


The Journal of Animal Ecology

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